26 



A NATURALIST IN THE GREAT LAKES REGION 



If we should make a«n ideal section of the earth at this early 

 stage (Fig. 22) the surface layer would be very irregular, due to 

 the indiscriminate infalling of planetesimals. The next zone 

 would be ver}' porous, similar to the contents ©f a basket filled 

 with large potatoes, only the fragments would be vastly larger, 



more or less cemented 

 with lava. The zone 

 below would be less 

 porous, as the planet- 

 esimal matter would be 

 pressed together by the 

 weight of the zone on 

 top of it. The central 

 core of the planet would 

 be very compact, due to 

 the pressure of the zones 

 above it. The diagram 

 shows the later rock 

 layers on the outside of 

 this early earth. 



]\Ieanwhile the young 

 earth, growing, reached 

 a stage when it doubtless 

 could hold the begin- 

 nings of an atmosphere. 

 As the earth's gravity 

 gradually increased it 

 would first be able to 

 hold the gases that are heaviest and least volatile. Carbon 

 dioxide is the heaviest of all the gases that make up our atmos- 

 phere. Ox}^gen is next in weight; then nitrogen, water vapor, 

 and hydrogen follow in succession. The earth is still not large 

 enough to hold much hydrogen. 



When the earth had reached such a size as enabled it to hold 

 water vapor the atmosphere was piled high with clouds. Rains 



Fig. 22. — Diagrammatic section of the earth 



